Minimizing compression set of curable rubbers

ABSTRACT

THE METHOD FOR REDUCING COMPRESSION SET OF CURABLE ELASTOMERIC MATERIALS COMPRISING HEATING THE CURED ELASTOMER TO A TEMPERATURE WITHIN THE RANGE OF 200* TO 400* F. FOR TIME WITHIN THE RANGE OF MORE THAN 72 HOURS AT 212*F. TO MORE THAN 1 HOUR AT 400*F. AND IN WHICH THE CURED ELASTOMER IS PREFERABLY AN EPDM RUBBER HAVING AT LEAST FIVE C=C GROUPS PER 1000 CARBON ATOMS BEFORE CURE.

Jan. 25, 1972 3,3?,17

MINIMIZING COMPRESSION SET OF CURABLE RUBBERS kK. H. WIRTH Filed Sept.8, 1969 UnitedStates Patent O "i 3,637,617 MINIMIZING COMPRESSION SET FCURABLE RUBBERS Kenneth H. Wirth, Baton Rouge, La., assignor toCopolymer Rubber & Chemical Corporation, Baton Rouge, La.

Filed Sept. 8, 1969, Ser. No. 855,940 Int. Cl. C08f 15/04 U.S. Cl.260-80.78 8 Claims ABSTRACT OF THE DISCLOSURE The method for reducingcompression set of curable elastomeric materials comprising heating thecured elastomer to a temperature within the range of 200 to 400 F. for atime within the range of more than 72 hours at 212 F. to more than 1hour at 400 F. and in which the cured elastomer is preferably an EPDMrubber having at least live C=C groups per 1000 carbon atoms beforecure.

This invention relates to the production of sulfur vulcanizable orcurable elastomers characterized by reduced compression set and itrelates more particularly to the treatment of sulfur vulcanizable andcurable EPDM rubbers having low compression set.

The invention will be described with reference to sulfur vulcanizableand curable EPDM rubbers. Briefly described, the term sulfurvulcanizable and curable EPDM rubbers, with which this invention findspreferable use, refers to rubbers formed by interpolymerization of amonomeric mixture containing ethylene, at least one higheralpha-monoolefin containing 3-16, and preferably 3-10, carbon atoms, asrepresented by propylene, butylene, pentylene and the like, and at leastone monomeric compound having two or more unsaturated carbon to carbonlinkages, such as a bridged or straight chained polyene, as representedby 1,4-hexadiene and the like straight chained dienes containing from2-18 carbon atoms, as described in U.S. Pat. No. 2,93 3,480, or a cyclicpolyene, such as dicyclopentadiene (DCP), 1,5-cyclooctadiene, 2-methyl(18, 4-9)tetrahydroindene and the like, but in which the polyeneis preferably selected of a bridged ring polyene, such as analkenyl-Z-norbornene, represented by 5-propionyl-Z-norbornene,S-butenyl-Z-norbornene and the like, and more preferably analkylidene-Z-norbornene such as 5-methylene-Z-norbornene,S-ethylidene-Z-norbornene and other 5-alkylidene-2-norbornenes in whichthe alkylidene group has from 2-10 carbon atoms.

Other bridged ring hydrocarbons include polyunsaturated derivatives ofbicyclo(2,2,2)octa2,5diene, polyunsaturated derivatives ofbicyclo(3,2,1)octane, polyunsaturated derivatives ofbicyclo(3,3,1)nonane, and polyunsaturated derivatives ofbicyclo(3,3,2)nonane. At least one double bond is present in a bridgedring of the above compound and at least one other double bond is presentin a bridged ring or in a side chain. Further examples ofpolyunsaturated bridged ring hydrocarbons and in the preparation of EPDMrubbers therefrom are `described in U.S. Pats. No. 2,933,480, No.3,093,620, No. 3,093,621 and No. 3,211,709. Halogen substituted bridgedring compounds, and especially chlorine substituted bridged ringcompounds, may be present in the monomeric mixture to be polymerized,examples of which are disclosed in U.S. Pats. No. 3,220,988 and No.3,222,330.

3,637,617, Patented Jan. 25, 1972 ICC The EPDM elastomers usuallycontain ethylene and the other monooleflin, such as propylene, in themole ratio of :20 and 20:80 with the bridged ring hydrocarboninterpolymerized in an amount to provide an unsaturation level greaterthan 1.5 C=C groups per 1000 carbon atoms of the elastomer, andpreferably 2.5-10 and more preferably 5-30 eifective CLC groups per 1000carbon atoms.

In instances where it is desired to prepare a tetrapolymer or a polymercontaining more than four different monomers, one or more of thealpha-monoolens containing 4-16 and preferably 4-10 carbon atoms may besubstituted for an equal molecular amount of bound propylene in thecomposition previously described. Such fourth monomer, when present,will normally be employed in an amount within the range of 5-20 molepercent, but smaller amounts to 1 mole percent can be used.

The polymerization is usually carried out in a solvent system in thepresence of a Ziegler type catalyst. The polymerization solvent may beany suitable inert or saturated hydrocarbon which is liquid andrelatively nonviscous under reaction conditions. Examples ofsatisfactory hydrocarbon solvents include open chain saturatedhydrocarbons containing 5-8 carbon atoms, as represented by hexane;aromatic hydrocarbons, preferably hydrocarbons containing a singlebenzene nucleus, such as benzene or toluene; and saturated cyclichydrocarbons which have boiling ranges approximating those for the openchain and aromatic hydrocarbons described, and preferably saturatedcyclic hydrocarbons containing 5 or 6 carbon atoms in the ringstructure. The solvent may be a mixture of one or more of the foregoinghydrocarbons, such as a mixture of aliphatic and naphthenic hydrocarbonshaving approximately the same boiling range as hexane.

As the Ziegler type catalyst, use can be made of any suitable Zieglertype catalyst known to the prior art, such as described in U.S. Pats.No. 2,933,480, No. 3,093,620, No. 3,093,621, No. 3,211,709 and No.3,113,115.

The preferred Ziegler type catalyst for use in the preparation of EPDMrubbers comprises the combination of a vanadium compound and an alkylaluminum halide. EX- amples of suitable vanadium compounds includevanadium trichloride, vanadium tetrachloride, vanadium oxychloride andvanadium acetylacetonate. Activators which are preferred include alkylaluminum chloride, having the general formulae R1AlCl2 and RZAlCl andtheir corresponding sesquichlorides in which R is an organic group suchas methyl, ethyl, propyl, butyl, isobutyl and the like. A catalystprepared from methyl or ethyl aluminum sesquichloride and vanadiumoxychloride is preferred with the optimum ratio usually within the rangeof 1 mole of vanadium oxychloride to 4-10 moles of alkyl aluminumsesquichloride t0 provide a ratio of aluminum to vanadium of 8:1 to20:1.

The EPDM rubbers are prepared in a conventional manner as described inThe Introduction to 'Rubber Technology, by M. Morton, ReinholdPublishing Corporation, New York (1959) Synthetic Rubber Technology,vol. I, by W. S. Penn, Maclaren and Sons, Ltd., London (1960); RubbenFundamentals of lts Science and Technology, by J. LeBras, `ChemicalPublishing Company, Inc., New York (1957); and Linear and StereoregularAddition Polymers, by N. G. Gaylord et al., 'Interscience Publishers,New York (1959).

Low compression set represents a very desirable characteristic for manyuses to be made of EPDM elastomers. When measured by the standard ASTMcompression set test, D-395-67, the compression set of commerciallyavailable EPDM elastomers varies from 90% to 30%, calculated as thepercent deformation which remains after the compressive forces have beenreleased from a set state and in which the value depends somewhat on thetype of third polyunsaturated monomer of which the interpolymer isformed and the amount of such polyunsaturated third monomer as well asthe compounding and conditions of cure. Lower compression set is securedwith interpolymers formed of third monomers such as the5-alkylidene-2-norbornenes, as represented by 5ethylidene2norbornene,and in which the amount of such third polyunsaturated monomer isincreased to provide higher eliective unsaturation levels in the uncuredor unvulcanized elastomeric material.

It is an object of this invention `to produce and to provide a methodfor producing sulfur vulcanizable and curable elastomers and preferablysulfur vulcanizable and curable EPDM interpolymers having reducedcompression set.

It has been found, in accordance with an important concept of thisinvention, that the amount of set which is retained upon compression ofa cured or vulcanized EPDM rubber is greatly reduced when the cured orvulcanized rubber is subjected to a post-heat treatment at a temperaturewithin the range of 212 to 400 1F. for a time ranging from a minimum of70 hours at the lower temperature to a minimum of l hour at the highertemperature and, more specilically, for a time of at least 70 hours at212 F., at least 24 hours at 250 F., at least 4 hours at 275-300 F., atleast l-2 hours at 350 F. and at least l hour at 400 F. Heat treatmentat a temperature in excess of 400 F. is undesirable because thermalbreakdown of the rubber occurs before achieving a desirable reduction incompression set. Heat treatment at temperatures below 200 F. appears toafford little if any beneficial effect on compression set, even whencontinued over an impractical period of time.

While the mechanism or reactions responsible for reduction incompression set in response to heat treatment of the type described havenot been established, it is believed that the activation of the rubber'molecules over the period of time for heat treatment enablesre-adjustment of the molecules to an oriented position withoutnoticeable change in density or shape of the cured elastomer to providean oriented rubber in which compression set is reduced to low levels bycomparison with the cured or vulcanized rubber. It is also believed thatcompression set is somewhat dependent upon crosslinked density of thecured elastomer with high compression set occurring with elastomers oflow cross-linked density in the state of cure. Heat treatment, inaccordance with the practice of this invention, appears to increasecross-linked density with corresponding reduction in compression setwithout change in composition of the cured or vulcanized rubber.Treatment, in accordance with the practice of this invention, is alsobelieved to cause some polysulde cross-links to become monosulde linksor shorter polysulde links.

The invention will hereinafter be illustrated by reference to thefollowing examples which are given by way of illustration, but not byway of limitation.

The elastomeric polymers of the various examples were compounded in thefollowing standard recipe:

Ingredient: Parts by weight Polymer 100 ISAF Black 80 Petroleumderivative plasticizer (Circosol 4240-Sun Oil Company) 55 Zinc oxide 5Stearic acid l Mercaptobenzothiazole 0.75 Tetramethyl thiuram monosulde1.5 Sulfur 1.5

Compression set was determined by the standard ASTM test at 158 F. for22 hours and/or at 212 F. for 22 hours, as indicated in the respectiveexamples.

EXAMPLES l to ll Polymer 1,000 ear- Ex. trade name Third Monomcr bonatoms Mooney 1 Royaleue 301.. Dicyclopentadiene 4. 2 28 2. Eujay4504.... 5inetliylene-2-norborne11e. 4.0 2S 3. Nordel 1440.1,4-hexadiene 6. 5 40 4. Enjay 477A. Ethylidene-2-norbornene. 3. 5 54 5Enjay 3501L... Methylene-2-norbornene... 3. 5 GG Royalene 502..Etliylidene-2-norborneno 5. 0 55 N ordel 1470-.. 1,4-l1exadiene 6. 5 70Epsyn 40- Etliylidene-Z-norb nene.- 2. 7 40 9. Ep 0.--.. 2.7 10.-..Epsyn 40A do... 5. 0 40 11.-.. Epsyn 55 .do 9.0 55

Compression set buttons were molded at 320 F. for 30 minutes. Heattreatment was carried out at the following temperatures and times:

70 hours at 158 F. 24 hours at 212 F. 48 hours at 212 F. 72 hours at 212F. 24 hours at 250 F. 48 hours at 250 F. 72 hours at 250 F.

The results, which are illustrated in FIG. l, indicate that markedreductions in compression set of the molded compounds is achieved afterheat treatment for 72 hours at 212 F. and after 24 hours at 250 F. withlittle if any additional benefit being derived by heat treatment formore than 72 hours at 250 F.

It Vwill also be noted that lowest values of compression set occur withpolymers having the higher number ot C=C groups per 1000 carbon atomswith the greatest reduction percentagewise being achieved with polymerswhich originally have higher compression set values. In any event,reductions ranging from 50% and more are achieved by post heat treatmentof the molded polymer in accordance with the practice of this invention.

EXAMPLES 12 to 15 Compression set tests were performed on the followingpolymers of ethylene, propylene and 5-ethylidene2 norbornene:

Epsyn 40 (40 Mooney, 2.7 C=C/l000 C) IEpsyn 40A (40 Mooney, 5.0 C=C/l000C) Epsyn 70 (70 Mooney, 2.7 C2G/1000 C) Epsyn 70A (70 Mooney, 5.0C=C/l000 C) Compression set test buttons were molded at 320 F. for l0minutes and for 30 minutes. Heat treatment at 275 F. and 300 F. wereapplied at various times ranging from 2 to 24 hours with the results setforth in the following Tables I to VIII.

TABLE I Aging Temperature 275 F.Epsyn 70 lolyiner Cured at 320 F 10cures 30 cures Compression set 22 hours at- TABLE VI Aging temperature300 F.Po1ymer EPsyn 70A Cured at 320 F 30' Cures 10 Cures 30' Cures 10Cures 032132292849090 211111142111112 s s T ss s nmmmmmmnmmmmmm 000mH000 mommhhhmmomhhh Nh 284N h 2004 248112 248112 630210100665008Mwwunlummww 1 s s 1 s s 1 1 maxaman@ OOOh-DhrmOOhh O hhh284 hhh28 N24811[1li t a s m 0 h 2 2 t e s n .m d F F e m a D 5 H n m 1 2 U 0 l C 5 m n0327971202321 0.2.30.0.1 2.9.5L&3.4 211111142211111 1 s n n mmmmmmmmwmmmmmwmmm Ohhh2oc4 Ohhh24 N248112 24812 t a s r u o h 2 2 t e s m dw F F we o C TABLE III Aging Temperature 275 F.-Polymer Elsyn 40 TABLEVII Aging temperature 300 P Polymer EPsyn 40 Cured at 320 F Cures 30Cures Cured at 320 F 10 Cures l Uuaged.

TABLE IV Aging Temperature 275 )Ff-Polymer EPsyn 40A TABLE VIII Agingtemperature 300 F.-Polymer EPsyn 40A 30 Cures Cured at 320 F 10 Cureswmnumnumwwuwm s e r u C /0 00 s e u c msm f umu 0 000 1. hhh

Hmm

{Il t a s m o h m Dac mi M 2 m o C C 5 0 5 4 5 5 l Unaged.

1 Unaged.

TABLE V Aging Temperature 300 F.Polymer EPsyn 70 It will be seen thatafter 2 hours post heat treatment at either 275 F. or 300 F.,compression set is reduced materially from the levels 0f the cured butuntreated 30 Cures 10 Cures polymers. No significant improvement incompression set is secured after 24 hours of heat treatment and bestresults are secured within the range of 2 to 12 hours of heat treatment.It will also be noted that compression set varies somewhat with curetime but that such differences level out with the heat treatment.

EXAMPLES 16 to 19 m 0 h A.: 2 8

Normal 1 66.4 Normal 1 Cured at 320 F Compression set 22 hours at- Thesame compounds were tested as in Examples 12 to 15 with heat treatmentto reduce compression set at teml Unaged.

8 peratures of 350 F. and 400 F. for times ranging from with littleimprovement being secured after two to four to 12 hours. The followingresults were secured: hours of heat treatment.

TABLE 1X It will be apparent from the foregoing that there is provided ameans for improving compression set of sulfur 5 curable polymers andparticularly EPDM type polymers whereby the compression set is reducedto desirable levels.

Aging temperature 350 F.

Compression set, 22 hours ats F. 212 F. It will be understood thatchanges may be made in the o details of formulation and operationwithout departing Cured 30 at 320 F..

EPsyn 70-H0urs aged: from the sp1r1t of the mvention, especially asdened 1n 10 the following claims.

is 232 I claim: 1. The method for reducing compression set of cured 17.42. or vulcanized elastomeric compositions comprising post- 14. 38treating the cured or vulcanized material by heating the 10. 18. 15cured or vulcanized elastomer to a temperature within the range of 200to 400 F. for a time within the range of at is. least 72 hours at 200 F.to at least 1 hour at 400 F. with the minimum time of 72 hours to 1 hourvarying with the post-treatment temperature within the range of 200 to400 F.

2. The method as claimed in claim 1 in which the curable elastomericmaterial is an EPDM elastomer.

3. The method as claimed in claim 2 in which the EPDM elastomer has aninitial level of unsaturation of at least 5 CIC groups per 1000 carbonatoms before cure. 4. The method as claimed in claim 1 in which thecured elastomeric product is heated for at least 72 hours at atemperature of about 212 F.

50 JOSEPH L. SCHOFER, Primary Examiner R. A. GAITHER, Assistant ExaminerTABLE X o 5. The method as claimed in claim 1 in which the curedAgintemlefatum 400 F- elastomeric product is heated for at least 24hours at a Compression set, 22 hours attemperature Of about 250 F.

1580141. 212 F 6. The method as claimed in claim 1 in which the curedelastomeric product is heated for at least 4 hours at Cured 30'215320"FJ a temperature of about 250 to 300 F d: EPSyn wwHoms age 24.0 56.9 7.The method as cla1med 1n claim 1 1n wh1ch the -g g3 cured elastomericproduct is heated for at least 1 to 2 304 205 hours at a temperature ofabout 350 F. g-g g2g 8. The method as claimed in claim 1 in which thecured elastomeric product is heated for at least 1 hour at a z 40temperature of about 400 F. 10.9 10.2 gg 3:2 References Cited 2&6 242UNITED STATES PATENTS g 3,203,937 8/1965 BreslOW et al 260-79.3 24:1 93,285,883 11/1966 Shepherd 26o-79.5 g 3,285,885 1l/1966 Shepherd et al260-79.5 33:2 5 3,461,105 8/1969 Anderson 260-75 17 5 7 3,502,641 3/1970Harrison et a1. 26o-94.9 G 260 3 25.4 s 25.9 4 30.1 1 30.3 s

At these higher temperatures of heat treatment, marked 260 9 5 88 2 SU'S C1- X'R reductions in compression set are achieved after one hour 74

